X-ray inspection apparatus for inspecting semiconductor wafers

ABSTRACT

An x-ray inspection system includes a cabinet including an x-ray source, a sample support supporting a sample to be inspected, and an x-ray detector. The system further includes an air mover configured to force air into the cabinet through an air inlet in the cabinet above the sample support. The air mover and cabinet are configured to force air through the cabinet from the air inlet past the sample support to an air outlet in the cabinet below the sample support. The cabinet may be constructed to provide an x-ray shield. The x-ray inspection system can be used in a clean room environment to inspect items such as semiconductor wafers.

FIELD OF THE INVENTION

The invention relates to x-ray inspection apparatus and in particular toan apparatus and method suitable for inspecting a semiconductor waferduring the processing of the semiconductor wafer to produce integratedcircuits. However, aspects of the invention relate to x-ray inspectionsystems in general and can be applied to systems for inspecting any typeof sample.

BACKGROUND TO THE INVENTION

Fabricating integrated circuits is a multiple-step sequence ofphotolithographic and chemical processing steps during which electroniccircuits are gradually created on a wafer made of pure semiconductor,typically silicon. The entire manufacturing process, from start tofinish, takes six to eight weeks and is performed in highly specializedfacilities referred to as fabrication plants. Fabrication plants requiremany expensive devices to function. Estimates put the cost of building anew fabrication plant over one billion U.S. dollars, with values as highas $3-4 billion not being uncommon. As a result, processing time in afabrication plant is very valuable. Any time that a fabrication plant isnot operating, for example for maintenance of a machine within theplant, is very undesirable.

So there is a need for all wafer processing steps to be extremelyreliable with minimal maintenance required, and also for all processingsteps to be made a quick as possible and to take a little space aspossible.

As part of making processes reliable and in order to ensure that thecircuits being produced operate properly, it is desirable to be able totest wafers for defects and faults at various stages of production.While optical inspection of surface features can be achieved rapidly andreliably, inspection of internal faults, such as voids, cracks andmisalignments in deposited conductive elements (such as through siliconvias, copper pillars and bumps) is more difficult. Current methods fordetecting these faults require taking a wafer out of the fabricationplant and testing using a focussed ion beam, scanning electronmicroscope or x-rays. However, as soon as a wafer is taken out of theclean environment of the fabrication plant it is effectively destroyedand can no longer be used.

It would be desirable to be able to accurately test semiconductor wafersfor voids, cracks and misalignments in deposited conductive elements, ina more efficient and non-destructive manner. It would also be desirableto be able to test semiconductor wafers for voids, cracks andmisalignments in deposited conductive elements quickly, in a manner thatdoes not lead to significantly increased processing time forsemiconductor wafers.

SUMMARY OF THE INVENTION

In a first aspect there is provided an x-ray inspection systemcomprising:

-   -   a cabinet, the cabinet containing an x-ray source, a sample        support for supporting a sample to be inspected, and an x-ray        detector; and    -   an air mover configured to force air into the cabinet through an        air inlet in the cabinet above the sample support,    -   wherein the air mover and cabinet are configured to force air        through the cabinet from the air inlet past the sample support        to an air outlet in the cabinet below the sample support.

The cabinet may be constructed to provide an x-ray shield, and may belead lined for this purpose. It is desirable from a safety perspectiveto minimise the escape of x-rays from the system.

An x-ray system of this type can be used in a clean room environment toinspect items such as semiconductor wafers. By maintaining a flow of airfrom above the sample support, past the sample support to an outletbelow the sample support, the sample can be protected from any dust anddebris. The system is configured such that the flow of air is maintainedthroughout the operation of the system, i.e. while samples are loaded,imaged, moved and unloaded from the system. The system is configured toprovide at least a Class 4 ISO 14644-1 cleanroom environment.

The x-ray source is preferably a sealed x-ray tube. Sealed x-ray tubesrequire much less maintenance than open x-ray tubes and so are bettersuited to use in a semiconductor fabrication plant where any time thatoperation of the plant is stopped for maintenance is very expensive.

The x-ray source preferably comprises a transmission target. The use ofa transmission target allows for a very small spot size x-ray source andallows for high magnification within a compact system because the samplebeing imaged can be brought close to the target. A sealed, transmissiontarget, x-ray tube is particularly advantageous for semiconductor waferinspection, as this type of x-ray tube can provide high resolutionimages, good reliability and long maintenance cycles.

The system is preferably configured to perform transmission microscopy.The x-ray detector may be configured to measure attenuation of a primaryincident x-ray beam from the x-ray source, to provide a two dimensionalimage of the sample or a region of the sample.

The system may further comprise a high performance air filter, such as ahigh-efficiency particulate absorption (HEPA) filter or ultra-lowpenetration air (ULPA) filter, the air filter being located above thesample support. An air filter of this type ensures that the air flowentering the cabinet through the inlet is free from dust.

In a preferred embodiment, the air mover is positioned above the samplesupport, and within the cabinet. In this embodiment, the air filter ispositioned between the air mover and the sample support. Providing theair mover within the cabinet allows for the production of a compactsystem. However, it is possible to place the air mover outside of thecabinet.

The system may comprise a plurality of air movers and a plurality of airfilters. Ideally the system is configured so that air flow within thecabinet is uniform and laminar and in a downward direction, without anyrecirculation of air. The number of air movers used can be chosen tosuit the geometry of the system components within the cabinet to achievelaminar flow. In a preferred embodiment, the system comprises two airmovers and two associated air filters.

The air mover may be a fan. The air filter may be coupled to the airmover. The air mover and air filter may be provided as a fan filterunit. The fan filter unit may comprise an enclosure having an air inlet,a fan within the enclosure and configured to draw air in through the airinlet, an air outlet and a filter plate spanning the air outlet so thatair exiting through the air outlet is forced through the filter plate.The fan filter unit may be configured to provide a higher pressurewithin the enclosure than external to the enclosure. Providing a higherpressure within the fan filter unit improves the uniformity of the airflow through the air filter, which is desirable to prevent any airrecirculation within the cabinet.

The fan filter unit may comprise a baffle plate coupled to the fan. Thebaffle plate is advantageously configured to provide for a uniform airflow through the filter plate. The fan may be located at a centre of theenclosure and the baffle plate may be configured to direct air from thefan to the extremities of the enclosure. The enclosure may have externalwalls, and the baffle plate may be configured to direct air from the fantowards the external walls.

The air mover may comprise an x-ray shield, such as a lead sheet,arranged to prevent the escape of x-rays from the cabinet through theair mover. The baffle plate within the fan filter unit may be an x-rayshield. It is desirable from a safety perspective to minimise the escapeof x-rays from the system.

The x-ray source is advantageously located above the sample support andis fixed relative to the cabinet. Placing the x-ray source above thesupport allows a sample on the top of the support, and in particular atop surface of the sample, to be brought close to the x-ray source. Thisis advantageous for providing high magnification images in a compactsystem. If the x-ray source is located above the sample it is alsoadvantageous for it to be stationary during operation to prevent thegeneration of any dust or debris from any movement mechanism that mightdamage the sample. It is also advantageous to keep the x-ray sourcestationary as it is a relatively bulky and massive component. It alsotypically requires very large power cables which are relativelyinflexible and difficult to move.

The x-ray inspection system may comprise a controller including an imageprocessor. The image processor may be connected to the x-ray detector toreceive data from the x-ray detector.

The x-ray inspection system may comprise a first positioning assemblythat allows for relative movement between the sample support and thex-ray source, wherein the positioning mechanism is located below thesample support. The first positioning assembly may comprise a firsthorizontal sample positioning mechanism for moving the sample support ina first horizontal direction, a second horizontal sample positioningmechanism for moving the sample support in a second horizontaldirection, and a vertical sample positioning mechanism for moving thesample support in a vertical direction.

In a preferred embodiment, the first positioning assembly is configuredso that the first horizontal positioning mechanism moves the samplesupport and the vertical positioning mechanism in a first horizontaldirection. The vertical positioning mechanism may be configured to moveboth the second horizontal positioning mechanism and the sample supportin the vertical direction. The first horizontal positioning mechanism ispreferably mounted directly to a supporting frame. This arrangement isadvantageous for a system that performs a raster scan of the sample inthe horizontal plane. The scan lines of the raster scan extend in thesecond horizontal direction so the second horizontal positioningmechanism is required to operate over the longest distance, frequentlyand fast. Accordingly the second horizontal positioning mechanism isconfigured to move only the sample support and not the mass of any ofthe other positioning mechanisms. The first horizontal positioningmechanism is also required to move fast and frequently compared with thevertical positioning mechanism. By mounting the first horizontalpositioning mechanism directly to a supporting frame, movement in thefirst horizontal direction can be made fast and accurate. The verticalpositioning mechanism alters the image magnification and is required tomove relatively infrequently, over a relatively shorter distance thatthe horizontal positioning mechanisms, and typically not at all during araster scanning operation. The vertical positioning assembly can be maderelatively less massive than the horizontal positioning mechanisms.

The supporting frame, to which the first horizontal positioningmechanism is fixed, may be mounted to the floor. In a preferredembodiment, the supporting frame comprises a first rigid sub-frameconfigured to be fixed to the floor, and a second rigid sub-framesupported on the first sub-frame through a damping mechanism, with thefirst horizontal positioning mechanism fixed to the second rigidsub-frame.

The x-ray inspection system may comprise a sample support positiondetection assembly comprising non-contact position measuring device,such as a laser interferometer, positioned adjacent to the samplesupport and configured to detect a position or change in position of thesample support. This is particularly advantageous for determiningprecisely the position of the sample within the horizontal plane, whichis required when producing very high magnification images and using themto produce a three dimensional model. In a preferred embodiment, thesystem comprises two non-contact position measuring devices, preferablylaser interferometers. A first non-contact position measuring device fordetecting changes in position of the sample support in the firsthorizontal direction and a second non-contact position measuring devicefor detecting changes in position of the sample support in the secondhorizontal direction. Of course, a pair of non-contact positionmeasuring devices could be arranged to detect changes in position indifferent directions within a horizontal plane than the first and secondhorizontal directions. Other possible non-contact position measuringdevices include optical linear encoders, magnetic encoders, capacitivesensors and sonar distance measuring devices.

The position information provided by the non-contact position measuringdevice or devices may be used by the image processor. In particular, thechange of position of the sample from image to image provided by thenon-contact position measuring device or devices can be used in atomosynthesis calculation. Precise positional information is requiredwhen producing a three dimensional model of very small features such asvoids in a semiconductor wafer, at very high magnification. The moreprecise the positional information for the sample, the better the imageresolution.

The x-ray inspection system may comprise a proximity sensor fixed to thex-ray source configured to provide a measurement of distance between thex-ray source and a surface of a sample on the sample support. Theproximity sensor may be a laser position sensor or a confocal sensor.The proximity sensor may be connected to the image processor to providedistance data to the image processor. The image processor may use themeasurement of distance from the proximity sensor in an image processingcalculation, such as a tomosynthesis calculation.

The controller may be connected to the sample positioning assembly andmay control the sample positioning assembly based on the measurement ofdistance provided by the proximity sensor. The proximity sensor providesan accurate distance measurement between the x-ray source and a topsurface of a sample, which can be used both in an image processingcalculation, such as a magnification calculation, and to prevent anycollision of the sample and the x-ray source. To provide a usefulinspection of small features in a semiconductor wafer in a compactsystem, the sample is brought very close to the x-ray source, but anycollision between the sample and the x-ray source would likely damageboth the sample and the x-ray source. Avoiding such collisions, whilebringing the sample very close to the x-ray source, is thereforenecessary.

The sample positioning assembly may comprise a linear encoder. Thecontroller may be configured to calibrate the linear encoder based onthe measurement of distance provided by the proximity sensor.

The x-ray inspection system may further comprise a second positioningassembly that allows for relative movement between the x-ray detectorand the x-ray source, wherein the second positioning mechanism islocated below sample support. The second positioning assembly maycomprise a horizontal detector positioning mechanism for moving thedetector in at least two non-parallel directions within a horizontalplane. The detector positioning mechanism may comprise a detectortilting mechanism configured to allow the detector to be tilted from thehorizontal plane about at least two non-parallel axes. The detector canthen be tilted so that an imaging surface of the detector is alwaysnormal to a line between a centre of the detector and the output spot ofthe x-ray source, wherever the detector is positioned. The controllermay be connected to the second positioning assembly.

The first and second positioning assemblies are advantageouslypositioned below the sample support. The ability to move the sample anddetector relative to the x-ray source allows for different portions ofthe sample to be imaged and different angles of inspection to be used. Aseries of images of the same sample, or the same portion of a sample,can be used in a tomosynthesis system to generate three dimensionalmodels and images of the sample and accurately locate and measurecracks, voids and other defects.

By locating a fixed x-ray source above the sample, and the positioningassemblies beneath the sample, all the moving components of the systemcan be located underneath the sample. With a downwardly directedairflow, this reduces the likelihood of any debris generated by themoving parts reaching and damaging the sample.

The system may comprise a perforated deck positioned below the x-raysource but above the first and second positioning assemblies. Theperforated deck and air mover are configured to provide a first airpressure above the deck and a second air pressure below the deck,wherein the second air pressure is lower than the first air pressure.The perforated deck is preferably positioned level with the height ofthe sample support when the sample support is in its uppermost position(which corresponds to maximum magnification). Even a small pressuredifference between the space above the sample and the space below thesample prevents any significant flow of air from below the sample toabove the sample.

The air inlet advantageously comprises a labyrinthine air flow path.This ensures that x-rays from the x-ray source cannot escape from thecabinet through the air inlet. The air outlet advantageously comprises alabyrinthine air flow path for the same reason. The air outlet ispreferably large to minimise any recirculation of air.

The sample support is configured to support the sample during x-rayinspection. In one embodiment, the sample support is configured tosupport a semiconductor wafer.

The sample support may be configured in accordance with one of theeleventh to fifteenth aspects of the invention, described below.

The ability to inspect semiconductor wafers for internal features in anon-destructive fashion during wafer processing is highly desirable. Byproviding a system in which air flow is directed through the cabinetfrom above the wafer to below the wafer, while the cabinet stillprovides the required x-ray shielding, this becomes possible.

The use of air filters, such as those found in fan filter units, aperforated deck level with the sample support, and the positioning ofthe x-ray source in a fixed position above the sample support, ensurethat clean room standards can be met. Further advantageous features,such as the use of a sealed transmission target x-ray tube, provide therequired reliability and image quality for a commercially attractivesystem.

In a second aspect of the invention, there is provided a method ofinspecting a semiconductor wafer comprising:

-   -   directing x-rays at the wafer;    -   detecting x-rays that have passed through the wafer; and    -   directing a laminar airflow from above the wafer, past the wafer        to below the wafer simultaneously with the steps of directing        and detecting.

The air flow preferably comprises HEPA or ULPA filtered air.

By providing a continuous flow of clean air past the wafer, with norecirculation of air that might have picked up dust or debris frommechanisms within the system, the inspection system can meet clean roomstandards and the risk of contamination of or damage to thesemiconductor wafer is minimised.

In a third aspect of the invention, there is provided an x-rayinspection system comprising: an x-ray source, a sample supportconfigured to support a semiconductor wafer to be inspected, and anx-ray detector; wherein the x-ray source is positioned above the samplesupport.

The x-ray source is preferably fixed to a supporting frame and does notmove during operation of the system. The sample support may bepositioned very close to the x-ray source to allow for the production ofhigh magnification images.

In this aspect, the x-ray inspection system may comprise a cabinet, thecabinet containing the x-ray source, the sample support and the x-raydetector; and an air mover configured to force air into the cabinetthrough an air inlet in the cabinet above the sample support, whereinthe air mover and cabinet are configured to force air through thecabinet from the air inlet past the sample holder to an air outlet inthe cabinet below the sample holder.

The sample support may comprise a support surface extending in ahorizontal plane and further comprise a sample support positioningassembly for positioning the sample support relative to the x-ray sourceor x-ray detector, the support positioning assembly being positionedbelow the sample support.

The first sample positioning assembly may comprise a verticalpositioning mechanism for moving the sample support in a verticaldirection, orthogonal to the horizontal plane, and a first horizontalpositioning mechanism for moving the sample support and the verticalpositioning mechanism in a first horizontal direction. The x-rayinspection system may further comprise a second positioning assemblythat allows for movement between the x-ray detector and the x-raysource, wherein the second positioning mechanism is located below thesample support.

In a fourth aspect of the invention, there is provided an x-rayinspection system comprising: an x-ray source, a sample support forsupporting a sample to be inspected, wherein the sample supportcomprises a support surface extending in a horizontal plane,

an x-ray detector; and a sample support positioning assembly forpositioning the sample support relative to the x-ray source or x-raydetector; wherein the sample positioning assembly comprises a verticalpositioning mechanism for moving the sample support in a verticaldirection, orthogonal to the horizontal plane, and a first horizontalpositioning mechanism for moving the sample support and the verticalpositioning mechanism in a first horizontal direction.

The sample positioning assembly may comprise a second horizontalpositioning mechanism configured to move the sample support in a secondhorizontal direction, non-parallel to the first horizontal direction,wherein the vertical positioning mechanism is configured to move boththe second horizontal positioning mechanism and the sample support inthe vertical direction.

The system may further comprise a controller connected to the first andsecond horizontal positioning mechanisms and configured to control thehorizontal positioning mechanisms to move the sample support to performa raster scan in horizontal plane relative to the x-ray source.Advantageously the scan lines extend in the second horizontal direction.The second horizontal positioning mechanism is required to operate overthe longest distance, frequently and fast. Accordingly the secondhorizontal positioning mechanism is configured to move only the samplesupport and not the mass of any of the other positioning mechanisms. Thefirst horizontal positioning mechanism is also required to move fast andfrequently compared with the vertical positioning mechanism. By mountingthe first horizontal positioning mechanism directly to a rigidsupporting frame, movement in the first horizontal direction can be madefast and accurate.

The system may be configured such that the vertical positioningmechanism has a shorter range of travel than the first and secondhorizontal positioning mechanisms. The system may be configured suchthat the vertical positioning mechanism operates to move the samplesupport more slowly than the first and second horizontal positioningmechanisms.

The vertical positioning mechanism alters the image magnification and istypically required to move relatively infrequently, over a relativelyshorter distance that the horizontal positioning mechanisms, andtypically not at all during a raster scanning operation. Because thevertical positioning assembly does not need to move as far or as fast asthe horizontal positioning mechanisms, the vertical positioning assemblycan be made relatively less massive than the horizontal positioningmechanisms.

The positioning assembly may comprise a plurality of motors. Inparticular, the first and second horizontal positioning mechanisms mayeach comprise one or more linear motors. The vertical positioningassembly may comprise a servo motor together with a lead screw. Thesystem may advantageously be configured to control the mechanisms withinthe positioning assembly to move the sample support to a plurality ofpredetermined imaging positions.

The x-ray inspection system advantageously further comprises a frameconfigured to be fixed to a floor, wherein the first horizontalpositioning mechanism is fixed to the frame. By fixing the firsthorizontal positioning assembly directly to a supporting frame, thefirst horizontal positioning assembly can be made fast and accurate. Theframe may be formed in two or more parts connected to one anotherthrough damping components to reduce vibration of the sample support.

The sample support may be configured to support a semiconductor wafer.The sample support may be configured in accordance with one of theeleventh to fifteenth aspects of the invention.

The x-ray source is preferably located above the sample support. Thex-ray source is preferably a sealed x-ray tube, with a transmissiontarget, as described in relation to the first aspect of the invention.

The x-ray inspection system may further comprise a sample supportposition detection assembly comprising a non-contact position measuringdevice, such as a laser interferometer, positioned adjacent to thesample support and configured to detect a position or change of positionof the sample support, as described in more detail with reference to thefifth aspect of the invention.

The x-ray inspection system may further comprise a proximity sensorfixed to the x-ray source for determining a distance between the x-raysource and a surface of a sample on the sample support, as described inmore detail with reference to the sixth, seventh and eighth aspects ofthe invention.

The system may be configured to perform a tomosynthesis calculationbased on images recorded by the x-ray detector.

The x-ray inspection system may comprise a detector positioning assemblyfor positioning the x-ray detector relative to the x-ray source, whereinthe detector positioning assembly comprises a horizontal detectorpositioning mechanism for moving the detector in at least twonon-parallel directions within a horizontal plane, and a detectortilting mechanism configured to allow the detector to be tilted from thehorizontal plane about at least two non-parallel axes. Advantageousfeatures of the detector positioning assembly are described in moredetail with reference to the ninth and tenth aspects of the invention.

In a fifth aspect of the invention, there is provided an x-rayinspection system comprising: an x-ray source, a sample support forsupporting a sample to be inspected, an x-ray detector; a samplepositioning assembly for positioning the sample support relative to thex-ray source or x-ray detector; a sample support position detectionassembly comprising a non-contact position measuring device positionedadjacent to the sample support and configured to detect a position orchange of position of the sample support; and an image processorconnected to the sample support position detection assembly.

The non-contact position measuring device may be a laser interferometer.The x-ray inspection system may further comprise a reflector mounted tothe sample support to reflect laser light from the interferometer.

The system may be configured to automatically move the samplepositioning assembly to a plurality of imaging positions, wherein theimage processor is configured to calculate a change in a position of thesample support from one imaging position to another based on an outputfrom the non-contact position measuring device. The image processor maybe configured to perform a tomosynthesis calculation on images recordedby the x-ray detector, using the output from the non-contact positionmeasuring device.

The sample support may comprise a support surface extending in ahorizontal plane. The sample positioning assembly may comprise a firstpositioning mechanism for moving the sample support in a firsthorizontal direction and a second positioning mechanism for moving thesample support in a second horizontal direction. The sample supportposition detection assembly may then comprise a first non-contactposition measuring device for detecting a position or a change ofposition of the sample support in the first horizontal direction and asecond non-contact position measuring device for detecting a position ora change of position of the sample support in the second horizontaldirection. The second non-contact position measuring device ispreferably a second laser interferometer and the x-ray inspection systemmay further comprise a second reflector mounted to the sample support toreflect laser light from the second interferometer.

In a system with two or more interferometers there may be two or morecorresponding laser light sources. Alternatively, the system maycomprise one or more beam splitters configured to split a laser lightbeam into two secondary beams which can then be used with differentinterferometers.

The sample positioning assembly may comprise a vertical positioningmechanism for moving the sample support in a vertical direction,orthogonal to the horizontal plane and wherein the sample supportposition detection assembly may comprise a third non-contact positionmeasuring device for detecting position or movement of the samplesupport in the vertical direction. The third non-contact positionmeasuring device may be positioned to detect a vertical position of thesample support or may be positioned to detect a vertical position of atop surface of a sample mounted on the sample support. The controllermay be configured to perform a magnification calculation based on anoutput of the third non-contact position measuring device.

The non-contact position measuring device or each non-contact positionmeasuring device may be a homodyne interferometer or a heterodyneinterferometer. Other possible non-contact position measuring devicesinclude optical linear encoders, magnetic encoders, capacitive sensorsand sonar distance measuring devices.

The x-ray source is advantageously located above the sample support. Thesample support may be configured to support a semiconductor wafer. Thesample support may be configured in accordance with one of the eleventhto fifteenth aspects of the invention.

The x-ray inspection system may comprise a proximity sensor fixed to thex-ray source and configured to determine a distance between the x-raysource and a surface of a sample on the sample support. An output of theproximity sensor may be connected to the controller. The proximitysensor may be as described in more detail in relation to the sixth,seventh and eighth aspects of the invention.

The x-ray inspection system may comprise a detector positioning assemblyfor positioning the x-ray detector relative to the x-ray source, whereinthe detector positioning assembly comprises a horizontal detectorpositioning mechanism for moving the detector in at least twonon-parallel directions within a horizontal plane, and a detectortilting mechanism configured to allow the detector to be tilted from thehorizontal plane about at least two non-parallel axes. Advantageousfeatures of the detector positioning assembly are described in moredetail with reference to the ninth and tenth aspects of the invention.

In a sixth aspect of the invention, there is provided an x-rayinspection system comprising: an x-ray source, a sample support forsupporting a sample to be inspected,

an x-ray detector, a sample positioning assembly including a firstpositioning mechanism for moving the sample support along a first axistowards and away from the x-ray source, a proximity sensor fixed to thex-ray source configured to provide a measurement of distance between thex-ray source and a surface of a sample on the sample support, and acontroller connected to the proximity sensor.

The controller may be connected to the sample positioning assembly andcontrol the sample positioning assembly based on the measurement ofdistance from the proximity sensor. The controller may comprise an imageprocessor and may use the measurement of distance from the proximitysensor in image processing calculations.

The proximity sensor may comprise a laser light source directing a laserbeam parallel to the first axis. The proximity sensor may be a confocalsensor.

Providing a direct measurement of the distance between the x-ray sourceand the top surface of a sample is beneficial for several reasons,particularly in a high magnification system in which the top surface ofthe sample, which typically comprises the regions of interest, isbrought very close to the x-ray source. First, the distance measurementcan be used to calibrate the first positioning mechanism so thataccurate positioning and subsequent image processing can be achieved.Second, the distance measurement can be used directly in a magnificationcalculation to provide an accurate measure of magnification. Third, thedistance measurement or multiple distance measurements can be used toprevent any collision between the top surface of the sample and thex-ray source, which would likely be very damaging to both.

Advantageously the controller is configured to calibrate the firstpositioning mechanism based on distance measurements from the proximitysensor. In particular, the positioning assembly may comprise a linearencoder arranged along the first axis and the controller may beconfigured to calibrate linear encoder using one or more distancemeasurements from the proximity sensor.

The controller may be configured to perform a magnification calculationusing the distance determined by the proximity sensor.

The positioning assembly may comprise a second positioning mechanismconfigured to move the sample support in a plane orthogonal to the firstaxis, and the controller may be configured to operate the secondpositioning mechanism so as to perform a scan of a top surface of asample on the sample support relative to the proximity sensor. The scanmay be a raster scan.

Advantageously, the controller is configured to record a closest pointof the sample recorded during the scan. The controller may then beconfigured to calculate a closest safe position of the first positioningmechanism from the x-ray source based on the closest point; and controlthe first positioning assembly to prevent the first positioning assemblyfrom being moved closer to the x-ray source than the calculated closestsafe position.

The x-ray source is advantageously located above the sample support. Thesample support may be configured to support a semiconductor wafer. Thesample support may be configured in accordance with one of the eleventhto fifteenth aspects of the invention.

The x-ray source is preferably a sealed x-ray tube, with a transmissiontarget, as described in relation to the first aspect of the invention.

The x-ray inspection system may further comprise a sample supportposition detection assembly comprising one or more laser interferometerspositioned adjacent to the sample support and configured to detect aposition or a change in position of the sample support, as described inmore detail with reference to the fifth aspect of the invention.

The x-ray inspection system may comprise a detector positioning assemblyfor positioning the x-ray detector relative to the x-ray source, whereinthe detector positioning assembly comprises a horizontal detectorpositioning mechanism for moving the detector in at least twonon-parallel directions within a horizontal plane, and a detectortilting mechanism configured to allow the detector to be tilted from thehorizontal plane about at least two non-parallel axes. Advantageousfeatures of the detector positioning assembly are described in moredetail with reference to the ninth and tenth aspects of the invention.

The system may be configured to perform a tomosynthesis calculation onimages recorded by the x-ray detector.

In a seventh aspect of the invention, there is provided a method ofcontrolling an x-ray inspection system, the x-ray inspection systemcomprising: an x-ray source; a sample support for supporting a sample tobe inspected, wherein the sample support comprises a support surface; anx-ray detector; a sample positioning assembly including a firstpositioning mechanism for moving the sample support along a first axistowards and away from the x-ray source and a second positioningmechanism configured to move the sample support in a plane orthogonal tothe first axis; and a proximity sensor fixed to the x-ray source fordetermining a distance between the x-ray source and a surface of asample on the sample support, the method comprising:

-   -   a) placing a sample on the sample support;    -   b) positioning the sample support using the first positioning        mechanism at a first position of the first positioning        mechanism;    -   c) moving the sample support in a plane orthogonal to the first        axis past the proximity sensor and recording the distance of a        surface of the sample from the x-ray source at a plurality of        positions as the sample support is moved in the plane;    -   d) calculating a closest safe position of the first positioning        mechanism from the x-ray source based on the recorded distances;        and    -   e) controlling the first positioning assembly to prevent the        first positioning assembly from being moved closer to the x-ray        source than the calculated closest safe position.

The method may further comprise performing a magnification calculationbased on the recorded distances.

The step of moving the sample support may comprise moving the samplesupport in a raster scan configuration.

In an eighth aspect of the invention, there is provided a method ofcontrolling an x-ray inspection system, the x-ray inspection systemcomprising: an x-ray source; a sample support for supporting a sample tobe inspected, wherein the sample support comprises a support surface; anx-ray detector; a sample positioning assembly including a firstpositioning mechanism for moving the sample support along a first axistowards and away from the x-ray source and a second positioningmechanism configured to move the sample support in a plane orthogonal tothe first axis; and a proximity sensor fixed to the x-ray source fordetermining a distance between the x-ray source and a surface of asample on the sample support, the method comprising:

-   -   a) placing a sample on the sample support;    -   b) positioning the sample support using the first positioning        mechanism at a first position of the first positioning        mechanism;    -   c) recording the distance of a surface of the sample from the        x-ray source at the first position; and    -   d) performing a magnification calculation based on the recorded        distance.

In this context a “magnification calculation” is a calculation of themagnification of an image of the sample, or a portion of the sample, onthe x-ray detector.

In a ninth aspect of the invention, there is provided an x-rayinspection system comprising: an x-ray source; a sample support forsupporting a sample to be inspected, wherein the sample supportcomprises a support surface extending in a first horizontal plane;

an x-ray detector; a sample positioning assembly for positioning thesample support relative to the x-ray source; a detector positioningassembly for positioning the x-ray detector relative to the x-raysource, wherein the detector positioning assembly comprises a horizontaldetector positioning mechanism for moving the detector in at least twonon-parallel directions within a second horizontal plane; and a detectortilting mechanism configured to allow the detector to be tilted from thesecond horizontal plane about at least two non-parallel axes.

The detector can then be positioned so that an imaging surface of thedetector is always normal to a line between the centre of the detectorand the x-ray source whatever the position of the detector in the secondhorizontal plane. Having the imaging surface of the detector alwaysdirectly facing the x-ray source in every imaging position provides thehighest quality of the resulting images, as it eliminates blurring thatoccurs when x-rays enter the detector at extreme oblique angles.

Advantageously, the two non-parallel axes are coplanar. The x-raydetector may comprise a planar imaging surface, and the two non-parallelaxes may also lie in the same plane as the imaging surface. Thisarrangement simplifies image processing calculations, particularly whenthe images are to be used in a tomosynthesis algorithm.

Advantageously, the tilting mechanism is driven independently of thehorizontal detector positioning mechanism. This allows for very accurateorientation of the detector. The tilting mechanism may comprise a firstgimbal and a second gimbal. In a preferred embodiment, the first gimbalis driven by a first gimbal motor and the second gimbal is driven by asecond gimbal motor. The first and second gimbal motors may beautomatically controlled by a single controller. The controller may beconfigured to control the first and second gimbal motors to position thex-ray detector in a plurality of imaging positions to generate aplurality of images that can be combined in a tomosynthesis calculation.The first and second gimbal motors may comprise direct read encoders onan output side.

The x-ray inspection system may further comprise a controller, thecontroller connected to and configured to control the detectorpositioning assembly, the controller configured to move the detector toa plurality of imaging positions and to control the tilting mechanism toensure that an imaging surface of the detector is normal (i.e.perpendicular) to a line between a centre of the detector and the outputspot of the x-ray source in each of the plurality of imaging positions.

The controller may be configured to control the horizontal detectorpositioning mechanisms to move the detector in a raster scan pattern ina horizontal plane.

The sample positioning assembly may comprise a vertical samplepositioning mechanism for moving the sample support in a verticaldirection, orthogonal to the horizontal plane. The sample positioningassembly may comprise a first horizontal sample positioning mechanismfor moving the sample support in a first horizontal direction, and asecond horizontal sample positioning mechanism for moving the samplesupport in a second horizontal direction. Advantageous features of thesample positioning assembly are described in relation to the fourthaspect of the invention. In particular, the second horizontal samplepositioning mechanism may be mounted on the vertical sample positioningmechanism and the vertical positioning mechanism mounted on the firsthorizontal sample positioning mechanism.

The sample support may be configured to support a semiconductor wafer.The sample support may be configured in accordance with one of theeleventh to fifteenth aspects of the invention.

The x-ray inspection system may further comprise a frame to which thesample positioning assembly and the detector positioning assembly aremounted, wherein the x-ray source is fixed to the frame.

The x-ray source is advantageously positioned above the sample support.

The system may be configured to perform a tomosynthesis calculation onimages recorded by the x-ray detector.

The x-ray source is preferably a sealed x-ray tube, with a transmissiontarget, as described in relation to the first aspect of the invention.

The x-ray inspection system may further comprise a sample supportposition detection assembly comprising a non-contact position measuringdevice, such as a laser interferometer positioned adjacent to the samplesupport and configured to detect a position or a change in position ofthe sample support, as described in more detail with reference to thefifth aspect of the invention.

The x-ray inspection system may further comprise a proximity sensorfixed to the x-ray source for determining a distance between the x-raysource and a surface of a sample on the sample support, as described inmore detail with reference to the sixth, seventh and eighth aspects ofthe invention.

In a tenth aspect of the invention, there is provided a method ofcontrolling an x-ray inspection system, the system comprising an x-raysource, a sample support for supporting a sample to be inspected,wherein the sample support comprises a support surface extending in afirst horizontal plane, an x-ray detector, a sample positioning assemblyfor positioning the sample support relative to the x-ray source, adetector positioning assembly for positioning the x-ray detectorrelative to the x-ray source, wherein the detector positioning assemblycomprises a horizontal detector positioning mechanism for moving thedetector in at least two non-parallel directions within a secondhorizontal plane, and a detector tilting mechanism configured to allowthe detector to be tilted from the second horizontal plane about atleast two non-parallel axes, the method comprising:

-   -   controlling the detector positioning assembly to move the        detector to a plurality of imaging positions and to controlling        the tilting mechanism to ensure that the detector is facing the        x-ray source in each of the plurality of imaging positions.

As described, having the imaging surface of the detector always directlyfacing the x-ray source in every imaging position provides the highestquality of the resulting images as blurring that results from x-raysentering the detector at an oblique angle is eliminated.

In an eleventh aspect of the invention, there is provided a samplesupport for a semiconductor wafer comprising:

-   -   a generally planar support surface having an imaging area        configured to support a semiconductor wafer; and    -   at least one depression in the imaging area of the support        surface in fluid communication with a vacuum port,    -   wherein the sample support has a thickness in a direction normal        to the planar support surface and wherein the rate of change of        thickness of the sample support has a maximum value of no more        than 5% per millimetre of travel across the imaging area.

In this context the term vacuum port means an outlet to which a vacuumsource can be connected. The application of a vacuum to the vacuum portcreates low pressure in the depression or depressions underneath a waferon the sample support, thereby holding the wafer in place. This is howconventional wafer chucks work.

The depression may have a sidewall. The sidewall preferably extends in acontinuous curve from a first side of the depression to an opposite sideof the depression. Advantageously, the sidewall has a minimum radius ofcurvature of at least 10 mm, and more preferably at least 15 mm. Theminimum radius of curvature is preferably at least one order ofmagnitude, and preferably at least two orders of magnitude, greater thanthe maximum depth of the depression below the planar support surface. Atransition region between the planar support surface and the sidewall ofthe depression may extend in a continuous curve and advantageously has aminimum radius of curvature of no less than 1 mm.

Preferably a maximum rate of change of depth of the depression withrelative to the planar support surface is no more than 0.2 mm per mm oftravel across the depression parallel to the planar support surface.

Advantageously, the thickness of the sample support varies by no morethan 10% of the maximum thickness across the imaging area, and morepreferably varies by no more than 5% across the imaging area.Advantageously, a minimum distance from a first side of the depressionto an opposite side of the depression is at least 10 times the maximumdepth of the depression and preferably at least 20 times the maximumdepth of the depression.

Advantageously, the sample support is formed from a homogenous,non-crystalline material, which does not give rise to significantcontrast variations in x-ray images of the sample support but ismechanically robust. Preferably, the sample support has a density ofless than 2000 kg/m³ and more preferably less than 1500 kg/m³. Suitablematerials include polyether ether ketone (PEEK), beryllium, and acetal.

The benefit of a sample support in accordance with this aspect of theinvention is that it does not give rise to significant contrast changesin x-ray images resulting from x-rays that have passed through thesupport. Advantageously, changes in the thickness of the wafer supportas a result of the depressions are gradual, are small compared to theoverall thickness of the support, and do not include any sharp edges.

The sample support may comprise a plurality of depressions within theimaging area. Each depression may be substantially annular. The radialwidth of each depression may be between 2 and 10 mm. The maximum depthof each depression below the planar support surface may be between 0.1and 0.5 mm.

The vacuum port may be positioned in an area outside of an imaging areaof the support.

In a twelfth aspect of the invention, there is provided a sample supportfor a semiconductor wafer comprising: a generally planar support surfacehaving an imaging area configured to support a semiconductor wafer; andat least one depression in the imaging area of the support surface influid communication with a vacuum port, wherein the depression has acurved sidewall that extends from a first side of the depression to anopposite side of the depression.

Preferably, the sidewall extends in a continuous curve from the firstside of the depression to the opposite side of the depression.

Advantageously, the sidewall has a minimum radius of curvature of atleast 10 mm, and more preferably at least 15 mm. The minimum radius ofcurvature is preferably at least 2 orders of magnitude greater than themaximum depth of the depression below the planar support surface.

In a thirteenth aspect of the invention, there is provided a samplesupport for a semiconductor wafer comprising:

-   -   a generally planar support surface having an imaging area        configured to support a semiconductor wafer; and    -   at least one depression in the imaging area of the support        surface in fluid communication with a vacuum port,

wherein a maximum rate of change of depth of the depression withrelative to the planar support surface is no more than 0.2 mm per mm oftravel across the depression parallel to the planar support surface.

In a fourteenth aspect of the invention, there is provided a samplesupport for a semiconductor wafer comprising:

-   -   a generally planar support surface having an imaging area        configured to support a semiconductor wafer; and    -   at least one depression in the imaging area of the support        surface in fluid communication with a vacuum port,

wherein the sample support has a thickness in a direction normal to theplanar support surface and the thickness of the sample support varies byno more than 10% of the maximum thickness across the imaging area, andmore preferably varies by no more than 5% across the imaging area.

In a fifteenth aspect of the invention, there is provided a samplesupport for a semiconductor wafer comprising:

-   -   a generally planar support surface having an imaging area        configured to support a semiconductor wafer; and    -   at least one depression in the imaging area of the support        surface in fluid communication with a vacuum port,

wherein a minimum distance from a first side of the depression to anopposite side of the depression is at least 10 times, and preferably atleast 20 times, the maximum depth of the depression.

In a sixteenth aspect of the invention, there is provided an x-rayinspection system comprising an x-ray source, an x-ray detector, and asample support in accordance with any one of the eleventh to fifteenthaspects positioned between the x-ray source and the x-ray detector.

The system may be configured to perform a tomosynthesis calculation onimages recorded by the x-ray detector.

The x-ray source may be located above the sample support. The x-rayinspection system may comprise a cabinet, the cabinet containing thex-ray source, the sample support and the x-ray detector; and an airmover configured to force air into the cabinet through an air inlet inthe cabinet above the sample support, wherein the air mover and cabinetare configured to force air through the cabinet from the air inlet pastthe sample holder to an air outlet in the cabinet below the sampleholder.

Features described in relation to one aspect of the invention may beapplied to other aspects of the invention. Any combinations of two ormore of the aspects of the invention are contemplated within thisdisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in detail, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of the basic components of an x-rayinspection system;

FIG. 2 is a first schematic cross-section illustration of an x-rayinspection system in accordance with the invention;

FIG. 3 is a second schematic cross-section illustration of the x-rayinspection system of FIG. 2 along plane A-A in FIG. 2;

FIG. 4 is a simplified perspective view of a system of the type shown inFIG. 2, with the cabinet removed;

FIG. 5a is a perspective view of the top of the cabinet of FIG. 4;

FIG. 5b is a cut-away view of the base of the cabinet shown in FIG. 4;

FIG. 5c is close-up view of the air flow through the outlet as shown inFIG. 2;

FIG. 6 is a schematic illustration of the arrangement of a samplepositioning assembly in the x-ray inspection system of FIG. 2;

FIG. 7 is a perspective view of a sample positioning assembly inaccordance with one aspect of the invention;

FIG. 8 is a schematic illustration of a sample support positiondetection system;

FIG. 9 is a perspective view illustrating the location of the positiondetection system of FIG. 8 in the sample positioning assembly of FIG. 7;

FIG. 10 is a schematic illustration of a sample proximity sensorassembly;

FIG. 11 is a perspective view illustrating the position of the proximitysensor in an x-ray inspection system of the type shown in FIG. 2;

FIG. 12 is a schematic diagram illustrating the distances used in aproximity calculation and in a magnification calculation;

FIG. 13 is a flow diagram illustrating a collision prevention operation;

FIG. 14 is a schematic cross-section illustration of an x-ray inspectionsystem in accordance with the invention showing a tilting mechanism forthe x-ray detector;

FIG. 15 is an illustration of a mechanism to move the x-ray detector inthe horizontal plane;

FIG. 16 is a perspective view of the detector tilting mechanism;

FIG. 17 is a perspective view of a sample support for a semiconductorwafer;

FIG. 18 is a schematic cross-section of a portion of the sample supportof FIG. 17;

FIG. 19 is a schematic cross-section of a portion of a typical waferchuck in accordance with the prior art;

FIG. 20 is a schematic diagram showing the control elements of the x-rayinspection system; and

FIG. 21 is flow diagram illustrating the operation of the x-rayinspection system in a continuous manner.

DETAILED DESCRIPTION

X-Ray Inspection System Components

FIG. 1 is a schematic illustration of the basic elements of an x-rayimaging system. The system shown in FIG. 1 comprises an x-ray source 10,which in this system is held stationary, a moveable sample support 12and a moveable detector 14. X-rays from the x-ray source 10 pass throughthe support and any sample mounted on the sample support, and impinge onthe detector 14. FIG. 1 illustrates the areas 16 on the sample supportcorresponding to the field of view of the detector 14. The field of viewof the detector is selected by a user by the relative positioning of thedetector 14, sample support 12 and x-ray source 10 so that the sample,or an area of the sample, is within the field of view of the detector.The detector can move to different imaging positions so that differentprojections can be taken through samples on the sample support. In thiscontext, a different projection means that the x-rays pass through thesample on the support in a different direction.

Typically there are 2 operation modes. In a first mode the detectorstays stationary and the sample support is moved to different locationsto acquire different fields of view. In a second mode, the detector andsample support are moved in a co-ordinated manner to get a differentangular projection through the same field of view. This coordinatedmotion enables three dimensional reconstructions to be generated usingtomosynthesis.

The support 12 is moveable in the XY plane in order that the sample onthe support can be moved to a position between the x-ray source and thedetector. In the example shown in FIG. 1, the support 12 is alsomoveable in a vertical or Z-direction. This allows the magnification ofthe detected image at the detector to be adjusted. In other words,larger or smaller areas of the support can be made to fall within thefield of view of the detector depending on the relative distance betweenthe x-ray source 10 and the support 12 and the x-ray source 10 and thedetector 14. As explained, the area of the sample to be imaged must fallwithin the field of view.

Generally, the x-ray source 10 includes a tube that generates the beamof x-rays by accelerating electrons from an electron gun and causing theenergetic electrons to collide with a metal target. The x-rays containedin the beam are sufficiently energetic to penetrate through thethickness of target objects on the sample support 12 so that attenuatedx-rays reach the detector 14. The differential levels of x-rayattenuation by the materials of different density in the sample andtheir different thicknesses, within the region being imaged producescontrast in the resulting image captured by the detector.

The detector 14 may be a digital detector and have a construction as iswell known in the art. Generally, the detector includes an active area,a sensor that converts the incoming x-rays over the active area intoanother signal type that can be measured or imaged, and an amplifierused to boost the amplitude of the signals. The signals are convertedfrom an analogue form to a digital form within the detector 14 and adigital image is output from the detector. An exemplary digital detectoris a complementary metal oxide semiconductor (CMOS) flat panel detectorthat includes a two dimensional pixel array of silicon photodiodesconstituting the active area.

Tomosynthesis

In FIG. 1, the detector 14 is shown in four different positions andthere are four corresponding regions 16 on the support. It should beunderstood that many more positions are possible. In a tomosynthesissystem, a three dimensional model of the area of the sample being imagedmay be constructed from any number of projections, and anything between12 and 720 projections is used in practice.

The resulting three dimensional model allows a user to inspect any planethrough the imaged area, and to review a three dimensional image to finddefects such as voids.

Various tomosynthesis algorithms and processing techniques are known inthe art, such as the ReconPro reconstruction solution offered by PrexionInc. of 411 Borel Avenue, Suite 550, San Mateo, Calif. 94402, USA.

A requirement for generating a three dimensional model using a pluralityof images is a knowledge of the precise spatial relationship betweenx-ray source, region of interest and detector for each image. The way inwhich the two-dimensional images are combined in tomosynthesis relies onthis geometric information, as it is required in the mathematicalformulas that are used.

Clean Room X-Ray Inspection System

In order to use a system as described above to inspect and generatedmodels of samples that are produced in a clean room environment, such assemiconductor wafers, during production, it is necessary that the x-rayinspection system itself meets clean room standards.

FIG. 2 is a schematic cross section of an x-ray inspection system inaccordance with one embodiment of the invention. FIG. 3 is a secondcross section of the system shown in FIG. 2, taken perpendicular to thecross section of FIG. 2 along plane A-A. FIG. 4 is a partially cut-awayperspective view of the system of FIGS. 2 and 3.

The system illustrated in FIGS. 2, 3 and 4 comprises an x-ray tube 100,a sample support 200 and a detector 300, within a cabinet 110. Thecabinet 110 is lead lined to provide a shield from x-rays generated bythe x-ray tube 100.

Within the cabinet 110 there is a supporting frame 120, on which thex-ray tube 100, sample support 200 and detector are all mounted. Thesample support 200 is configured to hold a semiconductor wafer (notshown in FIG. 2, 3 or 4).

The frame comprises a detector positioning assembly 310 (not visible inFIG. 3 or 4) that allows the detector 300 to move to in a horizontalplane. The detailed components of the detector positioning stage 310 arenot shown in the FIGS. 2 and 3 and any suitable arrangement may be usedfor a clean room compatible system. However, one inventive andadvantageous arrangement is described in detail with reference to FIGS.14-16. In this embodiment the detector positioning mechanism comprises afirst beam 314, extending along the X-axis, on which the detector issupported and the supporting frame 120 having a rail 318 extending inthe Y-direction, on which the first rail 314 is supported. Linear motorsare provided to allow the detector to be moved along the beam 314 and toallow the beam 314 to be moved along the rail 318. In this manner thedetector can be moved to any position within a horizontal plane (withinthe frame). A tilting mechanism is also provided that allows the activearea of the detector to be oriented to face the x-ray source in allimaging positions. The tilting mechanism is shown in FIG. 16 and will bedescribed in more detail later on in the specification. The detector inthis embodiment is a CMOS flat panel detector, such as the Dexela 1512NDT, available from Dexela Limited, Wenlock Business Centre, 50-52 WharfRoad, London, N1 7EU, United Kingdom.

A sample positioning stage 210 is provided on the supporting frame abovethe detector positioning stage 310. The detailed components of thesample positioning stage are shown in and described with reference toFIGS. 6 and 7. However, sample support positioning arrangements having adifferent geometry or layout may be used to provide a clean roomcompatible system. In this embodiment, the sample positioning stagecomprises a similar arrangement of motors and rails as the detectorpositioning stage for moving the sample support 200 in a horizontalplane, with the addition of a further motor to move the sample supportin a vertical direction, towards and away from the x-ray tube. Thisallows for the magnification of the images to be selected. The samplepositioning stage is described in more detail below with reference toFIGS. 6 and 7.

A sample input shutter 240 as shown in FIG. 3, is provided to allow asample, such as a semiconductor wafer, to be loaded onto the samplesupport 200. The shutter is provided in the cabinet and has apneumatically operated mechanism. The shutter is lead lined and sealedby steel labyrinth bars to prevent escape of x-rays. The shutter isautomatically opened when inspection of a wafer is complete to enableunloading of the completed wafer and reloading of a new wafer. The waferload/unload is handled by an Equipment Front End Module (EFEM) unit toprovide a continuous clean environment for the wafer as it is moved froma Front Opening Unified Pod (FOUP) to the sample support. A FOUP is aspecial plastic enclosure designed to hold silicon wafers securely andsafely in a clean room environment, and to allow the wafers to beremoved for processing or measurement by tools equipped with appropriateload ports and robotic handling systems. The EFEM used in thisembodiment is a Brooks JET™ Atmospheric Transport System unit, availablefrom Brooks Automation, Inc., 15 Elizabeth Drive, Chelmsford, Mass.01824, U.S.A.

The x-ray tube 100 is fixed to an x-ray tube bracket 115 and ispositioned above the sample support 200 and the detector 300. The x-raytube bracket 115 is provided on the frame 120 above the samplepositioning stage. The x-ray tube cannot move relative to the frame 120.

In this embodiment, the x-ray tube 100 is a sealed-transmissive type ofx-ray tube, such as the NT x-ray tube from Dage Holdings Limited, 25Faraday Road, Rabans Lane Industrial Area, Aylesbury, Buckingham HP198RYUnited Kingdom. This type of x-ray tube provides for a very long servicelifetime, typically more than 5000 hours of operation before maintenanceis required, as well as very high resolution imaging. Asealed-transmissive type of x-ray tube comprises a fully sealed vacuumtube and a transmission target forming a portion of the exterior wall ofthe tube. The transmission target is constructed so that electronsimpinge on a first side of the target facing towards the interior of thetube and at least some of the x-rays generated are emitted through asecond side of the target facing outwardly from the tube. This issometimes referred to as an end window transmission tube.

An end window transmission tube allows for the generation of an x-raysource with small spot size and allows the sample being imaged to bebrought close to the x-ray source. This means that high magnificationand high resolution images can be obtained. By arranging the x-ray tube100 above the sample support 200 and configuring the sample support tosupport a semiconductor wafer between the sample support and the x-raytube, the surface of the semiconductor wafer can be brought very closeto the x-ray source, allowing for high magnification images to beobtained within a compact system.

A pair of fan filter units (FFUs) 130 is mounted to the cabinet abovethe sample support. The FFUs are configured to draw air in throughrespective air inlets 132 in the ceiling of the cabinet and drive theair through a HEPA filter plate 134 in each FFU, downward past thesample support 200 to an air outlet 150 in the floor of the cabinet. Thedirection of the airflow is indicated by the arrows in FIGS. 2 and 3.

In this example, each FFU 130 comprises an enclosure 136, a fan 138configured to draw air into the enclosure through an air inlet 132 onone face of the enclosure and out through an outlet covered by a HEPAfilter plate 134. Each FFU is constructed so that when the fan 138 isrunning, the air pressure within the enclosure 136 is higher thanoutside the enclosure. This helps to provide a uniform flow of airthrough the filter plate 134, and minimises local flow rate variations.

In this embodiment each FFU 130 also comprises an internal shield 140positioned between the fan and the filter plate, which is not a featureof standard FFUs. This shield 140 has two functions. It is both an x-rayabsorber and an airflow baffle. However, two separate components couldbe used, one for each of these functions. The shield 140 is a lead linedsteel tray that is larger than and spans the air inlet 132 of the FFU130 so that x-rays from the x-ray tube 100 cannot escape through the airinlet 132. The airflow path past the shield 140 and out of the FFU 130is made labyrinthine. The shield also forces air from the fan 138 to theouter edges of the enclosure 136, as is clearly illustrated in FIGS. 2and 3. This promotes a uniform air flow through the filter plate 134.

The air flow through the cabinet from the filter plates 134 past thesample support 200 is laminar. There is no recirculation of air frombelow the sample support 200 to a position above the sample support. Theuse of HEPA filters 134, laminar airflow and ensuring that there are nomoving parts in the system between the filter plates 134 and the samplesupport 200, means that there is minimal risk of any dust or otherparticulates from becoming airborne, or falling from above, and landingon, and thereby contaminating or damaging, the semiconductor wafer onthe sample support 200.

In order to ensure that there is no recirculation of air from below thesample support 200 to above the sample support, a perforated deck 160(best seen in FIG. 4) is provided at the level of the sample support.The perforated deck 160 is positioned at the same height as an uppermostposition of the sample support 200, corresponding to the highestmagnification of the system. The perforated deck 160 allows air fromabove the deck to pass to beneath the deck, but creates a small pressuredifference across the deck so that there is higher air pressure abovethe deck than below the deck. The perforated deck restricts the air flowpath of the air by approximately 50%. This restriction increases thepressure of the air above the deck. This small pressure differencesubstantially prevents recirculation of air to above the deck, as airwill naturally flow from an area of high pressure to an area of lowpressure. A baffle plate 165, as shown in FIG. 3, is also provided topromote laminar air flow.

The air outlet 150 in the floor of the cabinet is relatively largecompared with the air inlets 132, again to promote laminar airflow andreduce any recirculation of air upward. FIG. 5a shows the top of thecabinet. The air inlets 132 can be seen, allowing air to be drawn intothe FFUs. FIG. 5b is a cut-away view of the air outlet in the bottom ofthe cabinet. The air outlet 150 comprises four separate openings. Theopenings are surrounded by steel blocks 154. The spaces between the fouroutlet openings are provided to accommodate electrical cabling forproviding power and data to and from the cabinet. The air outlet iscovered by an outlet shielding plate 152 (shown in FIG. 5b transparentand in dotted outline) to prevent the escape of x-rays from the cabinet.The function of the outlet shielding plate 152 is best shown in FIG. 5c. The outlet shielding plate 152 is lead lined to absorb x-rays. Steelblocks 155 are also provided on the underside of the shielding plate asx-ray absorbers within the air flow path to the outlet. The steel blocks154 and 155 absorb x-rays and are positioned so that the air flow pathto the outlet is made labyrinthine. This ensures that no x-rays canescape the cabinet through the air outlet.

In operation the FFUs 130 continuously force air through the cabinet toensure that no dust or debris produced as a result of the operation ofthe sample positioning assembly and detector positioning assembly canreach samples on the sample support 200. The FFUs provide for at least aclass 4 ISO 14644-1 clean room environment within the cabinet. The FFUsoperate throughout the movement and operation of the sample support 200and detector 300, and as the samples are loaded and unloaded from thesystem. A system as described with reference to FIGS. 2, 3 and 4 shouldbe able to operate for thousands of hours without requiring maintenance.

Sample Positioning

An exemplary sample positioning assembly will now be described in detailwith reference to FIGS. 6 and 7.

The sample positioning assembly is used to position samples relative tothe x-ray source so that images of desired regions of interest withinthe sample and different projections of regions of interest can beobtained. The x-ray source is positioned above the sample support and isstatic. It is therefore desirable to be able to move the sample supportin the XY plane, which is the horizontal plane, to provide differentprojections and images of different regions of the sample. It is alsodesirable to move the sample support in the Z-direction, which is thevertical direction, towards and away from the x-ray source, to alter themagnification of the images. In particular, for semiconductor wafersthere is a need to bring the wafer very close to the x-ray source sothat very high magnification images can be produced while keeping theoverall height of the system within a standard ceiling height andallowing the system to be reasonably easily transportable.

In operation, when obtaining a set of different projections for atomosynthesis calculation, the sample needs to be moved in the X- andY-directions more often than in the Z-direction. Once the imagemagnification is set for a sample, then the sample need only be moved inthe X- and Y-directions to obtain the different projections.

In order that the x-ray inspection process does not become a bottleneckwithin a wafer processing plant, the inspection process needs to befast. This means that the mechanism for moving the sample in the X- andY-directions needs to be fast. It also needs to be accurate,particularly at very high magnification, in order to produce highresolution three dimensional models, as discussed.

In this embodiment, the sample positioning assembly is configured tomove in raster scan pattern between imaging positions as a line by linecollection of images along parallel lines, with these parallel scanlines extending in the X-direction. The X direction is indicated by theX arrow shown in FIGS. 6 and 7. This means that X-direction movementmechanism will have the highest amount of travel, and therefore shouldbe fast so as to increase the overall speed of the process forcollecting the x-ray images that are needed. For this reason, in thisembodiment, the X-axis drive mechanism is coupled directly to the samplesupport, which means that the X-axis drive mechanism moves minimal masscomprising only the sample support and the moving portions of x-axisdrive mechanism. Most importantly, the X-axis drive mechanism does notsupport or carry either the Y-axis drive mechanism or Z-axis drivemechanism and this enables the X-axis drive mechanism to move morequickly. Because the Z-axis drive mechanism is used relativelyinfrequently and does not need to be as fast as the X-axis and Y-axisdrive mechanisms, it can be made with a relatively lower mass than theX-axis and Y-axis mechanisms. The Z-axis drive mechanism is positionedto support the X-axis drive mechanism and moves both the X-axis drivemechanism and the sample support in the Z-axis. The Y-axis drivesupports the Z-axis drive mechanism and so moves the Z-axis drivemechanism, the X-axis drive mechanism and the sample support in theY-axis direction. The Y-axis drive mechanism is mounted to a supportingframe, to which the x-ray source is also mounted.

This arrangement is illustrated schematically in FIG. 6. The samplesupport 200 is mounted on a shuttle 212. The shuttle moves in theX-direction on a first frame 214. The shuttle 212 and frame togetherform the X-axis drive mechanism. The first frame 214 is mounted on atrack 216 and can move up and down the track 216. The track 216 andfirst frame 214 together form a Z-axis drive mechanism. The track 216 ismounted on a second frame 218 and can move along the second frame 218 inthe Y-direction.

FIG. 7 illustrates one embodiment of this arrangement in more detail, ina system as shown in FIGS. 2, 3 and 4. In FIG. 7 it can be seen that thesample support 200 is configured to support a circular semiconductorwafer. The sample support is mounted to a first pair of linear motors220. Each linear motor comprises a track of permanent magnets 224extending in the X-direction and a coil assembly 222 that travels alongthe track in response to electrical control signals. Linear motors ofthis type are available from Aerotech, Inc., 101 Zeta Drive, Pittsburgh,Pa. 15238, USA. The linear motors 220 are mounted on a first frame 214.The first pair of linear motors 220 on the first frame form part of theX-axis drive mechanism.

The first frame 214 is mounted on a pair of lead screws 230, on oppositesides of the first frame 214, only one of which is visible in FIG. 7.Each lead screw 230 is driven by a rotary motor 234 through an angulargearbox 238. The lead screws 230 are each mounted to a plate 218 andmove the first frame 214, together with the X-axis drive mechanism, upand down in the Z-axis direction relative to the plates 218. Four guiderails 216 are provided at the corners of the first frame 214 to supportthe first frame 214 and keep it stable as the first frame is moved inthe Z-axis direction by the lead screws 230. The lead screws 230,associated rotary motors 234, gear boxes 238 and guide rails 216 formpart of the Z-axis drive mechanism. A linear encoder 236 is providedbetween the first frame 214 and the plate 218 to determine and allowcontrol of the vertical position of the first frame 214 and wafersupport 200.

The plates 218 slide along guides 242 formed on the supporting frame120. A second pair of linear motors 244 is connected between the plates218 and the supporting frame 120 to move the plates 218, together withthe Z-axis drive mechanism and the X-axis drive mechanism, in theY-direction relative to the supporting frame. The second pair of linearmotors may be larger and of a higher power than the first pair of linearmotors, as they are required to move a greater mass than the first pairof linear motors. Linear motors of this type are available fromAerotech, Inc., 101 Zeta Drive, Pittsburgh, Pa. 15238. USA. The secondpair of linear motors 244 on the first frame 120 forms part of theY-axis drive mechanism.

It should be clear that although this arrangement has been described inrelation to a system for inspecting semiconductor wafers in a clean roomenvironment, it can also be used in x-ray inspection systems that do notneed to operate in a clean room environment and so do not include theair movers and air filters described.

Sample Position Measurement

As explained, one of the requirements for producing good qualitytomosynthesis models is very accurate knowledge of the relative positionof the x-ray source, sample and detector. In particular, it is necessaryto know accurately the change in relative positions from one imagingposition to the next so that the images can be properly combined.

To provide high magnification images, the distance between the sampleand the x-ray source is much smaller than the distance between thedetector and the x-ray source. This means that small changes in positionof the sample lead to large changes in the image recorded by thedetector. This in turn means that the position of the sample needs to beknown to a much higher accuracy than the position of the detector.

A non-contact position measuring device may be used to accuratelydetermine the position of the sample support. In one embodiment of theinvention, an interferometer based system is used to determine thechanges in position of the sample support from one imaging position tothe next. FIG. 8 is a schematic illustration of the interferometer baseddetection arrangement. Two interferometers are provided. A firstinterferometer 256 is used to determine changes in the position of thesample support 200 in the X-direction and a second interferometer 258 isused to determine changes in the position of the sample support in theY-direction. The assemblies for the X- and Y-directions are identical.Each assembly comprises a laser light source 260, 262 that provides alaser beam to the associated interferometer 256, 258. The interferometerdirects a portion of the laser beam to a mirror 252, 254 mounted to thesample support 200. Light reflected from the mirror is directed back tothe interferometer and then to a detector 264, 266. Changes in theinterference between light that has travelled to the sample support andlight that has not travelled to the sample support, as the samplesupport is moved, is detected at the detector to provide a very accuratemeasure of the change in position of the sample support in the sensed(i.e. x or y) direction. Suitable interferometer systems, including thelaser light sources and mirrors, are available from Renishaw plc, NewMills, Wotton-under-Edge, Gloucestershire, GL12 8JR, United Kingdom.Other possible non-contact position measuring devices include opticallinear encoders, magnetic encoders, capacitive sensors and sonardistance measuring devices.

The output from the detectors is an accurate measure of the change inposition of the sample support in the X- and Y-directions as the samplesupport moves between imaging positions. These measurements are providedto an image processor, as will be described, and used in a tomosynthesiscalculation. The measurements from the detectors may also be used tocalibrate the X and Y positioning assemblies.

FIG. 8 illustrates an arrangement in which a separate laser light sourceis provided for each interferometer. However, it should be clear that asingle laser light source and a beam splitter could be used.Furthermore, it is possible to use an identical arrangement to determinechanges in position of the sample support in the Z-direction, althoughthe position of the sample support in the Z-direction typically does notneed to be determined with such a high degree of accuracy.

FIG. 9 illustrates how the arrangement as illustrated in FIG. 8 isintegrated into the sample positioning assembly shown in FIG. 7. Thefirst mirror 252 is fixed to the sample support 200 facing in theX-direction and the second mirror 254 is fixed to the sample supportfacing in the Y-direction. The interferometers 256, 258 are fixed to thesupporting frame 120. The mirrors 252, 254 have sufficient height andwidth that light from the interferometers 256, 258 is incident on themirrors in all possible imaging positions. In this example, the usablearea of each mirror is 320 mm wide and 20 mm high. The mirrors aretypically provided as part of the interferometer system as describedabove. However, suitable mirrors are also available as standalone itemsfrom optics manufacturers, such as Gooch and Housego PLC, Dowlish Ford,Ilminster, TA19 0PF, UK.

It should be clear that although this arrangement has been described inrelation to a system for inspecting semiconductor wafers in a clean roomenvironment, it can also be used in x-ray inspection systems that do notneed to operate in a clean room environment and so do not include theair movers and air filters described. It may also be used in conjunctionwith a different arrangement for positioning the sample and thedetector.

Proximity Measurement

As described, for high magnification images it is necessary to bring thesample very close to the x-ray source. It is therefore necessary tocontrol the position of the sample in the Z-direction reliably. It isalso necessary to know the position of the sample in the Z-direction forimage processing and data interpretation purposes.

While the position of the sample support 200 in the Z-axis can bedetermined from the Z-axis positioning mechanism or from a linearencoder mounted to the Z-axis positioning mechanism, there is theproblem that different samples have different thickness and so theactual distance between the x-ray source and a top surface of the samplecannot be determined accurately from the position of the sample support200. Accordingly, in one aspect of the invention, a proximity sensor isused to provide a direct measurement of the distance between a topsurface of the sample and the x-ray source.

Providing a direct measurement of the distance between the x-ray sourceand the top surface of a sample is beneficial for several reasons,particularly in a high magnification system in which the top surface ofthe sample, which typically comprises the regions of interest, isbrought very close to the x-ray source. First, the distance measurementcan be used to calibrate the Z-axis positioning mechanism, so thataccurate positioning and subsequent image processing can be achieved.Second, the distance measurement can be used directly in a magnificationcalculation to provide an accurate measure of magnification. Third, thedistance measurement or multiple distance measurements can be used toprevent any collision between the top surface of the sample and thex-ray source, which would likely be very damaging to both.

FIG. 10 is a schematic illustration of a proximity sensing arrangementin accordance with an aspect of the invention. A semiconductor wafer 20is illustrated on a sample support 200, beneath a sealed x-ray tube 100.A laser distance sensor 400 is fixed to the x-ray tube 100. A suitablelaser distance sensor is available from Keyence Corporation, 1-3-14,Higashi-Nakajima, Higashi-Yodogawa-ku, Osaka, 533-8555, Japan. As analternative, a confocal detector may be used. A laser beam from thedistance sensor is indicated by arrow 401. The sample positioningassembly 210 is illustrated schematically and includes a linear encoder236 for determining a Z-axis position of the sample support 200. Boththe sample positioning assembly and the distance sensor are connected toa controller 500.

FIG. 11 illustrates the mounting of the laser distance sensor 400 to thex-ray tube 100 in a system of the type shown in FIGS. 2, 3 and 4.

The laser distance sensor 400 provides a direct measurement to the topsurface of the sample, in this example a semiconductor wafer. The laserdistance sensor 400 measures the distance to the sample from its outputend, herein referred to as the read head facing the top surface of thesample. The x-ray tube 100 produces x-rays from an output spot on thetransmission target. The transmission target forms the output window 101of the x-ray tube, so that the output spot lies in the plane of theoutput window 101 of the x-ray tube 100. The read head of the laserdistance sensor 400 may not be mounted at exactly the same height as theoutput spot of the x-ray tube 100. In other words, the read head of thelaser distance sensor may not be coplanar with the output window of thex-ray tube. But the difference in height between the output window andthe output end of the laser distance sensor, known as the offset, can becalculated during system set-up by imaging a feature of known size, ortwo features of known spacing, on the sample support, in differentpositions as explained below

FIG. 12 is a schematic illustration of the arrangement shown in FIG. 11,showing how the offset between the read head 402 of the laser distancesensor 400 and the output window 101 of the x-ray tube can be calculatedby imaging a pair of features 280, 281 of a known spacing from oneanother on a gauge plate on the sample support 200.

The known distance between the features 280 and 281 on the gauge plateis D₁. The distance between the images 380, 381 of the two features 280,281 on the detector 300 is D₂. D₂ can be determined from the output ofthe detector using standard image processing techniques.

It is well known in this field that the ratio D₁/D₂ is equal to theratio A/H. The distance H between the output window of the x-ray sourceand the imaging surface of the detector 300 is known from the systemspecifications. So A can be calculated using the formula:

A=Hx(D ₁ /D ₂).

The distance B between the read head of the laser distance sensor andthe sample support is directly measured by the laser distance sensor400. Consequently, the offset C between the read head of the laserdistance sensor 400 and the output window of the x-ray tube isdetermined by subtraction:

C=B−A.

Since, A=Hx(D₁/D₂), C=B−(Hx(D₁/D₂)).

Therefore, the difference in height, or offset C, between the outputwindow and the read head of the laser distance sensor can be calculatedfrom the formula C=B−Hx(D₁/D₂) during system set-up by imaging a featureof known size.

Subsequent measurements of the distance to the sample from the read headof the laser distance sensor can be adjusted by this offset C to get thedistance from the output window to the sample, which is used inmagnification calculations as explained below.

The laser distance sensor can then be used to calibrate a height sensorwithin the sample positioning assembly. In this example, the heightsensor is the linear encoder 236, which is used in the Z-axis samplepositioning mechanism, as shown in FIG. 10. This is particularlybeneficial if, during inspection, the sample is positioned so that it isobscured from the laser distance sensor by the x-ray tube so that thelaser distance sensor measurements cannot be used directly. When thesample is in a position that it is not obscured by the x-ray tube,referred to here as the starting position, the controller 500 receivesdistance measurements from the laser distance sensor 400 and at the sametime receives an output from the linear encoder 236. The absolutedistance between the top plane of the sample and the output window ofthe x-ray tube can be established using the laser distance sensor 400.This is done by taking the absolute measurement reading from the laserdistance sensor 400 and subtracting the offset C, calculated asdescribed. This distance between the top plane of the sample and theoutput window of the x-ray tube is then used as the calibration for thelinear encoder, which measures changes in distance from the startingposition. This calibration process can be carried out periodically.

In addition, the laser distance sensor measurements can be used todetermine image magnification, which is used during image processing.Image magnification (IM) is the ratio of the size of the object as itappears in the image on the detector 300 to the actual size of anobject. With reference to FIG. 12, it is known in this field that themagnification ratio is equal to H/A. A is determined from the measureddistance B minus the calculated offset C. H is a known value. So imagemagnification is given by the formula:

IM=H/A=H/(B−C).

For example, if the distance H is 350 mm, the measured distance B is 12mm and the offset value C has been calculated as 2 mm, then the imagemagnification will be:

IM=H/(B−C)=350/(12−2)=35.

This means that a distance between features that appears as a 35 mmdistance on the detector is an image of a real distance of 1 mm.

This ability to accurately determine magnification has two benefits.Firstly, the size of the features within the sample can be establishedvery accurately, allowing good quantitative assessment of geometricfeature sizes such as wafer bump diameter or void area. Secondly, duringtomosynthesis, the angle and location of individual projections is wellknown, so the computed three-dimensional model can be made accurate. Animage magnification calculation using measurement from the laserdistance sensor is typically carried out as a calibration calculationbefore a set of images of a particular region or regions of interest arecaptured.

It should be clear that although this arrangement has been described inrelation to a system for inspecting semiconductor wafers in a clean roomenvironment, it can also be used in x-ray inspection systems that do notneed to operate in a clean room environment and so do not include theair movers and air filters described.

Collision Prevention

The proximity sensor, or laser distance sensor, 400 illustrated in FIGS.10, 11 and 12 can also be used to prevent any collision between thex-ray source and the sample. A collision between the x-ray source andthe sample is likely to irreparably damage the sample and also causesignificant damage to the x-ray tube. Because samples, in this examplesemiconductor wafers, can have different thicknesses, simply relying onan output from the linear encoder 236 which only provides informationabout the position of the sample support, may not be effective atpreventing collision when trying to obtain very high magnificationimages, which require that the sample be placed very close to the x-raytube, making the distance A in FIG. 12 very small.

In order to prevent collision, prior to inspection of a semiconductorwafer at high magnification, the wafer is safely raster scanned by thelaser distance sensor at a low magnification height, which is known tobe safe for all possible wafers, to establish the distance of thetop-most feature on the wafer 20 from the end of the x-ray tube 100. Theraster scan is achieved by operating the sample positioning assembly tomove the sample support in the XY plane. This illustrated in FIG. 10.The location on the linear encoder scale is recorded for the lowmagnification height as a baseline value. The shortest distance measuredby the laser height sensor, corresponding to top-most feature on thewafer 20, is recorded. The controller 500 then creates a virtualreference plane based on the baseline encoder scale reading at themeasured shortest distance which dictates how close the wafer can safelybe brought to the surface of the x-ray tube. The virtual plane may belocated at a predetermined clearance above the top-most feature on thewafer. For example, if the shortest distance to the top of the samplemeasured by the laser distance sensor at the low magnification height is12 mm and the known offset between the output end of the laser distancesensor 400 and the output window of the x-ray tube, C, is 2 mm, then theshortest distance between the output window of the x-ray tube and thetop of the sample during the raster scan is 10 mm. If it is desired thatin operation the top surface of the sample should be no closer than 1 mmfrom the output window of the x-ray tube in order to prevent anypossibility of a collision, then the sample support can be moved nocloser than 9 mm from the low magnification height recorded as abaseline on the linear encoder. This 9 mm maximum travel can becontrolled using the linear encoder readings. The linear encoder scalecan be used during inspection of the wafer to ensure that no collisioncan occur.

FIG. 13 is a flow diagram illustrating the control process used toprevent collision between the x-ray tube and the wafer. In step 450 asemiconductor wafer is loaded into the system. In step 460, the wafer ispositioned below the laser distance sensor at a known safe height, i.e.at a height at which even the very thickest sample would be well clearof the x-ray tube. In step 470 the laser distance sensor is thenactivated and the sample support moved in a raster scan pattern in theXY plane by the sample positioning assembly. The distance to the closestfeature on the wafer is recorded. In step 480 the maximum permittedupward travel of the sample support is calculated corresponding to aminimum safe distance between the sample support and the x-ray tube or amaximum safe height for the sample support. In step 490 inspection ofthe wafer is performed at high magnification but with the sample supportat a position lower than or equal to the maximum safe height for thesample support.

This process can be performed quickly and automatically for every newsample that is loaded into the machine. Again, it should be clear thatthis system and method is applicable not only to semiconductor wafersbut to any type of sample that is required to be imaged at highmagnification.

Detector Positioning

As described, the x-ray detector is positioned below the sample supportto capture x-rays that have passed through the sample. The detector is aflat panel detector that includes a two dimensional pixel array ofsilicon photodiodes, as previously described.

In order to record different projections through a sample, the detectormust be moved accurately to different imaging positions. The projectionsare then combined using a tomosynthesis algorithm to generate athree-dimensional model of the sample or of a region of the sample. Asdescribed, it is desirable for the plurality of different projections tobe recorded as quickly as possible. And for high magnification images,in which the sample support is positioned very close to the x-raysource, the x-ray detector must move much greater distances betweenimaging positions than the sample support, and so it is necessary forthe detector to be moved at a relatively higher speed than the samplesupport.

In order that the detector can be moved accurately but at a high speedit is advantageous that the detector be moved within the XY plane, onrigid axes. The alternative of moving the detector on a pivotable,arcuate track, which has been used in prior systems, does not allow forsuch accurate movement at high speed because the mechanism is lessrigid. This alternative system would also suffer from excessivevibration when starting and stopping at high speed. Movement of both thedetector and sample within parallel XY planes, without any Z-axismovement, also has the advantage that the image magnification remainsthe same for all imaging positions, as the magnification is determinedby the formula IM=A/H as previously discussed.

However, movement of a detector having the detector face that lies inthe XY plane solely within that XY plane suffers from the disadvantagethat the detector is not always facing the x-ray source. At extremeoblique angles between the face of the detector and the point ofemission of the x-rays from the x-ray source blurring of the image canoccur. In one aspect of the invention, a tilting mechanism is providedfor the detector, in addition to a mechanism for moving the detector inan XY plane, which allows the detector to be oriented so that it facesthe x-ray source in all imaging positions.

FIG. 14 is a schematic cross-section illustration of an x-ray inspectionsystem in accordance with the invention, as illustrated in FIG. 2 butshowing a tilting mechanism 320 for the x-ray detector. The detectorpositioning assembly 310 allows the detector 300 to move in a horizontalplane.

The remaining features of the system shown in FIG. 14 are as describedwith reference to FIG. 2. The cabinet and FFUs are configured for cleanroom operation. A sample positioning stage 210 is provided on thesupporting frame above the detector positioning stage. The detailedcomponents of the sample positioning stage are shown in, and describedwith reference to, FIGS. 6 and 7. The x-ray tube 100 is fixed to thebracket 115 and is positioned above the sample support 200 and thedetector 300.

In order to provide for high speed movement in the XY plane, first andsecond linear motors are used to move the detector in the x directionand y direction respectively. As illustrated in FIG. 15, a first linearmotor 312 is mounted between a frame 326 of the detector assembly and ahorizontally extending beam 314. The beam 314 extends in the X-directionand is supported to move along the supporting frame 120. To provide forthis movement, a pair of second linear motors 316 (only one of which isvisible in FIG. 15), extending in the Y-direction, is mounted on thesupporting frame 120 and drive the remote ends 315 of the beam 314.Activation of the first linear motor 312 moves the detector assemblyalong the beam 314 in the X-direction. Activation of the second linearmotors 316 moves the beam 314 and the detector assembly in theY-direction, which is perpendicular to beam 314. This mechanism allowsthe detector to be moved within a horizontal plane quickly andaccurately. The first and second linear motors may be identical and arelarger that the linear motors used for the sample support positingassembly. Linear motors of this type are available from Aerotech, Inc.,101 Zeta Drive, Pittsburgh, Pa. 15238. USA.

FIG. 16 shows the tilting mechanism 320 in detail. Two further axes ofmovement for the detector are provided by this tilting mechanism 320 sothat the planar imaging surface, or face 305, of detector 300 is alwaysable to be faced directly at the x-ray source. The detector 300 ismounted to a first gimbal frame 322 so that it can be tilted about afirst rotary axis 321. A first motor 324 is configured to rotate thedetector about the first rotary axis. One example of a suitable motor isthe APR150DR-135 from Aerotech Inc., 101 Zeta Drive, Pittsburgh, Pa.15238, USA.

The first gimbal frame 322 is rotatably mounted to a motor 328, which ismounted to second gimbal frame 326, to rotate about a second rotary axis327. The second motor 328 is configured to rotate the first gimbal frame322 about the second rotary axis 327. The second gimbal frame 326 ismounted to the mover of linear motor 312 and the stator of linear motor312 is mounted to the beam 314, as described with reference to FIG. 15,so that the detector and tilting mechanism can be move along the beam314. The first and second motors comprise direct read encoders on anoutput side to allow for a very accurate determination of theorientation of the detector.

The tilting mechanism is arranged so that the first and second rotaryaxes 321, 327 are coplanar with the active area, or face 305, of thedetector. This means, as shown in FIG. 16, that both first rotary axis321 and second rotary axis 327 run across, or align with, the face 305of the detector 300. This simplifies the image processing calculationsrequired. It also removes the need for any compensatory Z-axis movementof the sample support or detector to address changes in imagemagnification. This is because the centre of the face of the detectorremains at the same Z-axis distance from the x-ray source in allpositions and so the image magnification remains constant regardless ofthe position of the tilting mechanism.

The first and second rotary motors are connected to and controlled by acontroller (not shown in FIG. 16) that is also connected to the linearmotors for the XY movement of the detector. The controller is configuredto ensure that in each imaging position in which the detector is stoppedto record an x-ray image, the active area 305 of the detector is facingthe x-ray source. Preferably, the controller will orient the activearea, or face 305, of the detector so that it is perpendicular to a linedrawn from the centre of the face of the detector to the point at whichthe x-rays are emitted from the x-ray source.

By constantly facing the active area 305 of the detector towards thex-ray source, oblique angles between the face of the detector and thepoint of emission of the x-rays from the x-ray source, which can causeblurring of the image, are avoided. This improves the quality of theimage.

It should be clear that although this arrangement has been described inrelation to a system for inspecting semiconductor wafers in a clean roomenvironment, it can also be used in x-ray inspection systems that do notneed to operate in a clean room environment and so do not include theair movers and air filters described. It is also possible to use thisdetector positioning assembly with a different arrangement for thesample support position assembly and without the position detectionsystems so far described.

Wafer Chuck

The sample support 200 for the semiconductor wafers 20 holds eachsemiconductor wafer in position by applying suction to a rear surface ofthe wafer. This is a well known wafer handling technique that avoidsdamage to the wafer. FIG. 17 is a plan view of the sample support andshows a plurality of concentric depressions or grooves 610 formed in aplanar upper support surface 612 of the sample support. The grooves 610are connected to a vacuum port 620 by a radial channel 614. When a waferis placed onto the support surface 612 a vacuum is applied to the port620. This holds the wafer firmly against the support surface 612. The“imaging area” of the sample support is the portion of the samplesupport that may appear in an x-ray image of a portion of thesemiconductor wafer.

In an x-ray inspection system as described, x-rays from the x-ray tube100 not only pass through the semiconductor wafer but must also passthrough the sample support 200 before reaching the detector 300. Thesample support must therefore be made from a material that does notattenuate the x-rays to too great an extent and does not have acrystalline structure that would diffract x-rays. Suitable materialsinclude polyether ether ketone (PEEK), beryllium, and acetal.

However, even these materials will attenuate the x-rays to some extent.The amount of attenuation depends on the thickness of the sample supportthat the x-ray must pass through. The grooves 610 result in a localthinning of the sample support and so the pattern of thickness changescaused by the grooves will appear in the resulting x-ray images. Thegroves of a conventional wafer chuck are rectangular in cross-sectionwith parallel sidewalls and a flat bottom, as shown in FIG. 19. FIG. 19shows a wafer chuck 280 with a rectangular cross-section groove 680. Ifthe x-ray image spans a groove, x-rays that pass a groove on the waferchuck will be less attenuated than those that pass through the remainderof the wafer chuck. The rectangular cross-section means that, given thevertical sidewalls of the grooves, the change in wafer chuck thicknessis very abrupt. This can cause very sudden changes in contrast in thex-ray image that would obscure or confuse the image, particularly ofweakly contrasting features of a wafer. This in turn would makeautomated analysis of the images slow or even impossible in some cases.

To minimise this problem, rather than providing grooves havingrectangular cross-sections, the grooves or depressions used in thepresent invention for providing suction to the rear of a wafer areconfigured to provide only small and gradual changes in wafer chuckthickness to cause less patterning in the x-ray image that would obscureor confuse the image. Thus, the thickness of the wafer chuck iscontinuously varied rather than having sudden changes in thickness, andthe grooves are made comparatively wide and shallow.

FIG. 18 is a schematic cross-section of one of the grooves 610 of FIG.17 in accordance with the invention. In this example the groove has awidth W of around 4.88 mm and a maximum depth below the support surfaceof 0.2 mm. The sidewall of the groove extends in a continuous curve froma first side to a second side of the groove. The curve is substantiallycircular and has a maximum radius of curvature of around 16 mm. It canbe seen that the radius of curvature is substantially two orders ofmagnitude greater than the maximum depth of the groove. This ensuresthat the rate of change of thickness of the wafer check is small. Themaximum thickness of the sample support in the imaging area D is 5 mm.

The size and shape of the grooves must satisfy two competingrequirements. The grooves must be large enough to provide a sufficientsuction force on the back of the wafer. But they must also not obscureor confuse images of features of interest in or on a semiconductorwafer.

In this example, the depth of the groove increases to 0.2 mm over aradial distance of 2.44 mm, which corresponds to about a 4% change inthe thickness of the wafer since 0.2 mm/5 mm=0.04. The average rate ofchange of thickness of the wafer across the groove is 0.2/2.44=0.8 mmchange in thickness per mm of travel parallel to the planar supportsurface. The maximum rate of change of thickness of the wafer is at theedge of the depression and is approximately 0.165 mm per mm of travelparallel to the planar support surface.

The width of the groove W is two orders of magnitude greater than thetypical features of interest within a semiconductor wafer. As the groovevaries continuously across its width rather than having sharp edges,this means that at a magnification appropriate for inspection, onlyaround 1% of the total thickness variation across the groove is seen asvariation in the image background of a feature of interest.

When examining a 100 μm diameter solder bump on a silicon wafer,analysis software may use four points outside of the bump area todetermine a baseline for adjusting image contrast. In the worst case,these points will be 200 μm apart from each other. If the solder bumpsoverlie the edge of a groove, where the rate of change of thickness ofthe chuck is at its highest, the effective depth of the sample supportwill vary by about 0.66% across the imaged area. This is calculated asthe maximum rate of change of depth×diameter of region ofinterest/maximum thickness of chuck 0.165×0.02/5≈0.066%. This does notgive rise to a significant change in image contrast across the imagecompared to the contrast provided between the solder bump and itssurrounding area and any defects in the solder bump.

It is desirable not to have any sharp edges in the grooves ordepressions. In other words, the rate of change in the change ofthickness of the sample support should be low. This is to ensure thatthere are no sharp edges that might be enhanced by edge detectionalgorithms in the image processing software used. By providingdepressions that extend in a continuous curve from one side to another,sharp edges are avoided within the depression themselves. The edges ofthe depressions should also ideally be smooth. As can be seen in FIG. 18the edges 615 of the groove 610 are rounded to remove any sharptransitions. The radius of curvature of the transition region betweenthe sidewall of the groove and the planar support surface 612 has aminimum radius of curvature of around 2 mm.

It should be clear that this is just one example of a wafer support inaccordance with the invention and that different geometries for thegroves can be used that provide a low maximum rate of change ofthickness of the support across the imaging area. Clearly the requireddimensions of the grooves depend on the size and nature of the regionsto be imaged and on the density of the material (which is closelyrelated to how strongly x-rays are attenuated) of the sample supportcompared to the density of the samples being imaged.

System Operation

The various aspects of the x-ray inspection system so far described canbe controlled to operate automatically and synchronously with eachother. In particular the positioning of the sample support and of thedetector must be co-ordinated and informed by measurements from theposition detection arrangements. The air movers, x-ray tube and waferhandling equipment must also be co-ordinated with the positioningassemblies.

FIG. 20 is a schematic diagram illustrating the use of a centralcontroller 500, to co-ordinate the operation of each of the controllablecomponents of the system and to receive and process the data required toprovide a tomosynthetic model.

A central controller 500, including an image processor 510 is connectedto each of the controllable components of the system, as well as to anoutput 520 and a fabrication plant interface 530.

The controller 500 controls operation of the x-ray tube 100 as well asthe FFUs 130. It operates an automated wafer handling mechanism 540 thatextends through the shutter to place a wafer onto the sample support 200and also removes the wafer from the sample support 200 after the waferhas been inspected within the system. It positions the wafer through thesample support positioning assembly 210 and correspondingly controls thedetector through the detector positioning assembly 310. It receives theoutput from the detector to build the three-dimensional model. Itreceives input from the laser distance sensor 400 to control thevertical position of the wafer relative to the x-ray source to avoid acollision. Input from the laser distance sensor 400 is also used inimage magnification calculations. It also receives inputs from theinterferometers which indicate the change in position of the samplesupport 200 as it moves to different positions for images to becollected.

The controller coordinates the movement of the sample support and thedetector in accordance with a pre-programmed sequence of operation aswell as performing initial calibrations as previously described. Thecontroller must both control the sample stage, which has three axes ofmovement, and the detector, which has four axes of movement, two of thembeing rotational.

FIG. 21 is a flow diagram illustrating an exemplary cycle of operationfor an x-ray inspection system of the type illustrated in FIG. 20. Thesystem is activated in step 700. In this step, the controller isswitched on and the FFUs are subsequently switched on to establish alaminar air flow. The FFUs then run continuously during operation of thesystem. In step 710, the wafer handling assembly 540 is operated to loada semiconductor wafer into the system onto the sample support. The waferhandling assembly in this embodiment comprises conventional waferhandling equipment used in semiconductor fabrication plants, in thisexample the Brooks JET™ Atmospheric Transport System described earlier.The wafer is secured on the sample support using suction, as described.In step 720 the maximum height for the sample support is calculatedusing the process described with reference to FIG. 10-12. In step 730the x-ray tube 100 is activated and projections of a plurality ofdifferent regions of interest on the wafer are recorded by the detector300. In this process the sample support 200 and detector 300 are movedto a plurality of predetermined imaging positions. This process may berepeated for different regions of interest on the wafer as indicated byarrow 735. In step 740 the projections of a particular region ofinterest are processed using a tomosynthesis algorithm to generate athree-dimensional model. The image processor 510 uses the recordedimages from the detector as well as associated information about theposition of the sample, sample support and detector for each image. Thismodel is output to a display and/or memory in step 750, together withany other collected data relating to the image capture process. Afterall of the required images for a particular wafer have been captured bythe detector 300, the wafer is unloaded from the system in step 760using the wafer handling assembly 540, and new wafer can then be loadedin step 710. The unloading and loading of wafers can be carried outsimultaneously with the image processing operations.

The system can be integrated into a semiconductor fabrication plant.Automatic loading and unloading of semiconductor wafers to and from thesample support, at any desired point in the wafer processing operationand/or after wafer processing has been completed, can be achieved usingstandard wafer handling equipment. Control software for the positioningassemblies, detector and x-ray tube can be integrated with thefabrication plant control system for the controller 500.

1. An x-ray inspection system, comprising: a cabinet comprising an x-raysource, a sample support supporting a sample to be inspected, and anx-ray detector; and an air mover configured to force air into thecabinet through an air inlet in the cabinet above the sample support,wherein the air mover and cabinet are configured to force air throughthe cabinet from the air inlet past the sample support to an air outletin the cabinet below the sample support.
 2. The x-ray inspection systemaccording to claim 1, wherein the cabinet prevents x-rays from the x-raysource from escaping from the cabinet.
 3. The x-ray inspection systemaccording to claim 1, wherein the air mover is positioned above thesample support within the cabinet.
 4. The x-ray inspection systemaccording claim 1, wherein the air mover comprises an x-ray shieldarranged to prevent the escape of the x-rays from the cabinet throughthe air mover.
 5. The x-ray inspection system according claim 1, furthercomprising a HEPA or ULPA air filter, the air filter being located abovethe sample support.
 6. The x-ray inspection system according to claim 5,wherein the air filter is positioned between the air mover and thesample support.
 7. The x-ray inspection system according to claim 5,wherein the air filter is coupled to the air mover.
 8. The x-rayinspection system according to claim 7, wherein the air mover and airfilter are provided in a fan filter unit.
 9. The x-ray inspection systemaccording to claim 8, wherein the fan filter unit comprises a baffleplate coupled to the air mover.
 10. The x-ray inspection systemaccording to claim 1, wherein the x-ray source is located above thesample support and is fixed relative to the cabinet.
 11. The x-rayinspection system to claim 1, wherein the x-ray source is a sealed x-raytube with a transmission target.
 12. The x-ray inspection systemaccording to claim 1, further comprising a first positioning assemblythat allows for relative movement between the sample support and thex-ray source, wherein the first positioning assembly is located belowthe sample support.
 13. The x-ray inspection system according to claim1, further comprising a second positioning assembly that allows formovement between an x-ray detector and the x-ray source, wherein thesecond positioning assembly is located below the sample support.
 14. Thex-ray inspection system according to claim 1, further comprising aperforated deck positioned below the x-ray source and above the firstand second positioning assemblies, wherein the perforated deck and airmover are configured to provide a first air pressure above theperforated deck and a second air pressure below the perforated deck, andwherein the second air pressure is lower than the first air pressure.15. The x-ray inspection system according to claim 1, wherein the airoutlet comprises a shielding plate and a labyrinthine airflow path pastthe shielding plate to prevent the escape of the x-rays through the airoutlet.
 16. The x-ray inspection system according to claim 1, whereinthe sample is a semiconductor wafer.
 17. A method of inspecting asemiconductor wafer, the method comprising: directing x-rays at asemiconductor wafer; detecting x-rays that have passed through thesemiconductor wafer; and directing a laminar airflow from above thesemiconductor wafer, past the semiconductor wafer to below thesemiconductor wafer simultaneously with the steps of directing anddetecting.